In the process of permanent-waving, or "perming", of hair, the overall sequence may be expressed as follows:
1. Penetration of a reducing agent into hair
2. kSSk+RSH.revreaction.kSH+RSSk
3 kSSR+RSH.revreaction.kSH+RSSR
4. Rearrangement of protein chains.
5. Setting the curl by reforming cystine bonds.
It has been reported in the literature that under alkaline waving conditions, penetration is the slow step, with steps 2 and 3 presumably taking place as rapidly as penetration occurs. The fourth step is critical to the formation of a commercially acceptable curl. Under acid conditions, e.g., pH 5 to 7, it has been considered necessary to apply heat to achieve a proper curl. This is because step 2 is the dominant or limiting step. Under such conditions, the hair is highly susceptible to reformation of cystine bonds on rinsing through reversal of reaction 2.
Under alkaline conditions, the reaction proceeds well through step 3, with increased formation of cysteine (kSH) and concomitant loss of mixed disulfide (kSSR). It is for this reason that alkaline waving solutions have been favored by the beauty industry.
In conventional perming, therefore, the hair is contacted, after washing, with a waving solution of a reducing agent, normally a mercaptan, present in sufficient strength to cleave the cystine (disulfide) bond. Cleavage softens the protein structure (keratin) of the hair. Contact time depends on pH, the process used, and temperatures employed, which may be up to about 60.degree. C. Wetting agents or other penetrants are normally employed to aid reduction.
In practice, the key objective is to complete the waving process in less than one hour, with the safety and comfort of the client foremost in mind. Hair that has been chemically pretreated may be contacted with weaker solutions and for a lesser time than normal hair. Solutions of high reductant concentration may be required for hair difficult to treat, or to shorten process time.
The operator normally determines whether there has been adequate processing of the hair, or that maximum bond breakage has occurred, by formation of a test curl. This determination involves unrolling the mandrel a couple of curls to ascertain if a "soft S" at the diameter of the mandrel pattern has formed. This is a subjective but skilled determination of whether adequate softening of the hair has occurred, so as to produce a satisfactory wave, and is normally expressed by the operator as the time of contact of the waving solution with the hair. When adequate test curl has been achieved, the waving solution is normally rinsed from the hair, and while the hair is still set on mandrels, an oxidizing agent is applied to fix the new curl pattern. The hair is subjected to a final rinse, possibly with conditioning, then dried.